1. Field of Invention
The present invention relates to adjusting light intensity. The present invention is more particularly related to adjusting the light intensity of a polarized light beam and/or the intensity of light input to a kernel of a light engine or a light projection system.
2. Discussion of Background
An image can be specified by a variety of parameters that include Contrast Ratio (CR) and bit depth. In the following discussion, assume a bit depth of 8 bits. The image will contain 256 shades of gray. As illustrated in FIG. 1, Shade 255 is the maximum intensity bright state and shade 0 is the minimum intensity or blackest dark state that can be produced by the display. Note that the blackest dark state does not necessarily correspond to a condition of no-light in the image. Rather, it corresponds to the greatest reduction in input light intensity that the microdisplays can accomplish.
The CR is defined as: (Intensity of the bright state)/(Intensity of the dark state). The maximum CR of an image is (Intensity of shade 255)/(Intensity of shade 0).
Areas within a projected video image are made dark by driving corresponding microdisplay pixels to their darker shades of gray. That is, dark areas utilize only a portion of the full bit depth available from the microdisplay. The concept is illustrated, for example, by dark shades 110 in FIG. 1. A consequence is that there is a limited ability to render subtle shades of gray within dark areas. This effect is manifested as a “contouring” in the dark areas and is recognized as a visual artifact.
The limitation on rendering subtle shades of gray in dark areas may be addressed through the use of an adjustable iris. In this method, an iris is inserted into a light engine to control the amount of light incident on the microdisplays. When the content of the video image is bright, the iris is wide open and the corresponding image is bright. Within the bright image all 256 shades of gray can be utilized. When the content of the video image is dark, the diameter of the iris is reduced. In this way the brightness of the entire image is reduced.
The light levels of a projected image with an active light level input adjustment such as an iris are illustrated in FIG. 2. The microdisplays are still capable of modulating the full 256 shades of gray. Therefore, subtle shading of the dark state is possible. In addition, the blackness of the dark state has been reduced and the apparent CR 210 increases. The reason this is referred to as the apparent CR is that a different definition of CR is being used. Apparent CR=(Intensity of shade 255 in a bright frame)/(Intensity of shade 0 in a dark frame). Visual observation has demonstrated the validity of this definition of CR.
The conventional approach to implementing this type of light modulation scheme is to insert a mechanically adjustable iris into the light path of a light engine. Doing so may be accomplished without loss of intensity in the brightness of shade 255. In real time, an electronic circuit analyzes the content of the video image and an algorithm determines the appropriate light level and adjusts the diameter of the iris also in real time.